CODEWITHSUNDEEP
pythonpython-2.7
Steven Senko
Steven Senko

Reputation: 127

Why list is twice of its size?

With given list:

x = [x for x in range(10)]

Printing out indexes and values:

for i in range(-10, len(x)):
    print i, ": ", x[i]

The output is:

-10 :  0
-9 :  1
-8 :  2
-7 :  3
-6 :  4
-5 :  5
-4 :  6
-3 :  7
-2 :  8
-1 :  9
0 :  0
1 :  1
2 :  2
3 :  3
4 :  4
5 :  5
6 :  6
7 :  7
8 :  8
9 :  9

but

print x

returns:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Why is the actual list twice as long as the initialized list?

Upvotes: 1

Views: 329

Answers (3)

Ruo Chun Zeung
Ruo Chun Zeung

Reputation: 39

An array of 10 elements in python,

x of index     0   1  2  3  4  5  6  7  8  9
= x of index  -10 -9 -8 -7 -6 -5 -4 -3 -2 -1

see python 2.7 documentation tutorial

In your code:

for i in range(-10, len(x)):
    print i, ": ", x[i]

would have i run from -10 to len(x)-1, which in detail = -10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0, ..., 9.

That's 20 iterations in total, which explains why you get twice as len(x).

I guess what you want is:

for i in range(-len(x), 0):
    print i, ": ", x[i]

This would give normal size = len(x).

Upvotes: 0

Rahul Gupta
Rahul Gupta

Reputation: 47846

Python support negative indexes as well as positive indexes to access an element of a list.

In negative indexes, an element is accessed from the end of the list counting backwards.

So,
Element at index -1 is basically the last element of the list i.e. element at index 9.
Element at index -2 is the second last element i.e element at index 8.
..
..
Element at index -10 is the first element i.e element at index 0.

Your list size is not double. You are just printing out the same element twice when infact they are the same object, you are just using 2 different ways to access the same object.

You can check that they are the same using the is operator.

>>> x[0] is x[-10] # Both point to the same object
True

>>> x[1] is x[-9] # Both point to the same object
True
...
...
>>> x[9] is x[-1] # Both point to the same object
True

Upvotes: 6

Martijn Pieters
Martijn Pieters

Reputation: 1122172

You are using negative indexes. Those work too; in Python an index < 0 indexes from the end. So there are two different ways of indexing elements, counting from the start and from the end.

For example, the index 2 and -8 both refer to the 3rd element in x, so both indexes give you the value 2.

As such, the list is not twice as long, you are just printing the values twice; once for the negative index, and once again for the positive.

See note #3 under the sequence types operations table:

  1. If i or j is negative, the index is relative to the end of the string: len(s) + i or len(s) + j is substituted. But note that -0 is still 0.

or from the Introduction section of the Python tutorial:

Indices may also be negative numbers, to start counting from the right:

>>> word[-1]  # last character
'n'
>>> word[-2]  # second-last character
'o'

Upvotes: 12

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